1. Field of the Invention
The present invention relates to a device for monitoring the status of an industrial process. More particularly, the present invention relates to a resistor temperature device (RTD) trip apparatus for monitoring the thermal status of an industrial process, such as a nuclear reactor.
2. Description of the Prior Art
Industrial processes, such as the generation of electricity with a nuclear reactor, require monitoring of the various steps that define the process. Such monitoring is accomplished by including a sensor in proximity to the process step to be monitored. The sensor produces an electrical or mechanical signal that may be used to control the process or to indicate the status of the monitored process step.
In the example of a nuclear reactor, resistor temperature devices (RTDs) are often used to monitor thermal conditions at critical reactor locations. The RTD is typically an electrical device having a resistance value that varies in accordance with temperature. Variations in thermal conditions at various process locations are readily measured with an RTD.
The monitored thermal conditions are useful for many purposes. For example, a meter may be connected to the RTD to provide a continuous indication of temperature at the monitored process location. Amplifying and buffering circuitry may be added to the RTD to provide such temperature reporting at remote locations.
The RTD may be coupled to a trip circuit such that a signal output is produced when a preselected thermal condition is obtained. For example, a critical temperature may sound an alarm or may actuate process control devices. The RTD may also monitor thermal extremes at the monitored process location. In this way, a gross failure of the process, as indicated by excessively high or low temperatures, may actuate an alarm or initiate a system shutdown.
A disadvantage of the RTD (and other such sensors) is that as monitored conditions vary, the characteristic resistance value of the RTD varies in a non-linear fashion. As temperatures monitored by the RTD increase, the slope of the increasing RTD resistance decreases. Monitoring the resistance value of the RTD with an electronic circuit becomes quite difficult. A circuit set to sense a particular threshold level must include sophisticated and sensitive additional circuitry to be able to differentiate the various points on the RTD resistance curve.
An additional problem created by RTD non-linearity is that of operating a plurality of trip and monitor circuits from a single RTD. Each trip of metering circuit must incorporate additional complex circuitry to be able to interpret the varying slopes of the resistance curve produced by the RTD. In addition to adding expensive and delicate circuitry, the response time to changes at the RTD is significantly degraded. Most curve interpretations requires some sort of an integrator, such as a capacitor. Integration is a relatively slow process as compared to differentiation of a point on a curve having a constant slope. Integration requires incorporation of precision components, some of which require critical adjustments. This adjustment adds to the cost of manufacturing and reduces the reliability of the device, thereby necessitating increased maintenance.
In addition to providing more reliable, stable, and simple RTD operation, a trip unit based on a linear sensor input signal is more versatile. Offsets, calibrations, etc. may be easily incorporated into a linear circuit. Such options are much more difficult to include in a circuit providing a non-linear output signal.